Use the Nylon brush to finish off your ski and snowboard tuning.
If you are only going to have one brush- have Nylon brush as it is the most versatile.
If you are racing here is some advice from Dougie Crawford (World Cup Racer) :
"I use the nylon brush last, after brass and horsehair. After the other two can't get anymore wax out of the ski nylon often does. It's a finishing brush and gives the skis a good shine, Making your skis or board ready to fly!."
"The rectangular brush is great but if you are doing a lot of skis the oval one is bigger and makes it easier"